Prayer marks appear on certain areas of the skin due to continuous pressure from prayer positions. After reaching my early thirties I have developed noticeable black spots on my brown skin. These marks are not painful. Praying Muslims develop these marks on their foreheads and lower side of the left foot.
The condition is known in medical science as hyperkeratosis. It is the result of overgrowth of a protein named keratin which is responsible for the protection of the outer layer of the skin. Hyperkeratosis is the natural safety response of the body to protect skin subjected to excessive pressure ob bony areas.
Googles tells that this condition is treated by salicylic acid, an agent that breaks up the keratin and used to treat the scaly skin. Some sites suggest that shampoos have keratin agents in shampoos to scrap off the dandruff. Not sure if applying shampoo would improve it. Some treatments use hydroxyethyl urea to treat dryness of skin due to its water retention properties. I guess it would not be very useful in treating the prayer marks.
According to a sayings by the Holy Prophet Muhammad (Peace Be Upon Him) prayer-marks will shine for the Muslims on the day of the judgement. Therefore I have no worries about these marks. Some people try to make these marks more prominent and I would not like to be one of them. May Almighty Allah end my life as a slave of the Holy Prophet Muhammad (Peace Be Upon Him).